I know there is a window for socialisation experiences which closes when they are 16 weeks
OP, just wanted to clarify this - just so that it takes the pressure off you a bit. There is indeed a period during which dogs socialise to new people/animals/things more easily but it may help to realise it's not a window that shuts closed at 16 weeks. A dog can be socialised at any point during it's life - it's just that it accepts new experiences more readily the younger it is.
Please don't think of it as a 'now or never' scenario where if you don't introduce the dog to every type of experience before 16 weeks then you've missed your chance. Socialisation can still happen at a later date and, indeed, there is some truth in the saying that no experience is better than a bad experience when they are this young.
Lots of different, fun experiences at the dog's own pace is key...
In answer to some of your other questions:
On the first night I put him in the crate and put the crate by my bed. For the first 15mins he howled and I just sat outside the crate door repeating "settled down" in a soothing manner. When he settled I got into bed and repeated this all again but with my hand down/fingers poking into the crate. I kept saying "settled down, shush now" in a gentle voice and slowly withdrew my hand. It worked a treat and even months later I can use "settle down" as a cue for him to do so. He cried when he needed to toilet so woke me up.
I never used a lead in the garden and the puppy never legged off in the dark - in would take a very brave puppy indeed to do so. They are also pretty sleepy so not awake enough to scarper. In fact, letting them off lead even on walks, when very young is brilliant because they will literally stick like glue to your side and so gives them a good foundation for off lead walking. I let my springer off lead from about 12 weeks old and he would not be more than about 2 metres away from me. We spent lots of time with me running away from him, him catching me up and then getting lots of lovely treats. I understand it takes some bravery to do so - find a nice big, open field - but it is really worth doing.
p.s. Hope you are all enjoying d-day 